1. Field of the Invention
The present invention relates to an A/D conversion device which converts an analog input voltage to a digital value using a pulse delay circuit, which delays a pulse signal for a delay period according to a magnitude of the analog input voltage.
2. Description of Related Art
In the related art, as an A/D conversion device in which a high-resolution digital value is obtained with a simple configuration, as shown in FIG. 8, an A/D conversion device described in Japanese Unexamined Patent Application, First Publication No. H05-259907 is known. In an A/D conversion device 400 shown in FIG. 8, a pulse delay circuit 11 has a configuration in which a plurality of delay units (NAND1, BUF1, . . . , and BUF15) having various gate circuits are connected together in a ring shape. As the power supply voltage of each delay unit, an analog input voltage Vin which will be subjected to A/D conversion is supplied. If a sampling pulse (SP) is input to the pulse delay circuit 11, the SP sequentially passes through each delay unit with a delay period according to the power supply voltage, and revolves in the pulse delay circuit 11. The number of stages when the SP has passed through the delay units is determined by the delay period of each delay unit, that is, the analog input voltage Vin which is supplied as a power supply voltage. The pulse passage stage detection circuit 21 detects the number of passage stages (and the number of revolutions).
The output circuit 31 loads the detection result of the number of passage stages by the pulse passage stage detection circuit 21 at the same time that a latch pulse (LP) is input when the sampling time has elapsed. The sampling time has started after the input of the SP has started. The output circuit 31 outputs a value obtained by encoding the number of passage stages as a digital value (out) after A/D conversion.
In the A/D conversion device 400, when the analog input voltage Yin is within a predetermined input voltage range (Vmin to Vmax), the relationship between Yin and out becomes linear, as indicated by a line L10 of FIG. 9.
In the A/D conversion device 400, the delay time of the delay unit changes depending on environmental factors, such as temperature, the type of elements, a variation between elements, or the like. As indicated by a line L11 of FIG. 9, the slope (=resolution) of the input/output characteristic varies from the value of the line L10.
The changes will be described specifically. When the input/output characteristic is expressed by the line L10 of FIG. 9, the range in which the A/D conversion result with respect to a predetermined voltage range (Vmin to Vmax) is obtained becomes Δout0. Meanwhile, when the input/output characteristic is expressed by the line L11 of FIG. 9, the range in which the A/D conversion result with respect to a predetermined voltage range (Vmin to Vmax) is obtained becomes Δout1. The sampling time is identical. Since the line L10 and the line L11 are different in slope, as shown in FIG. 9, Δout0 and Δout1 which are the ranges of the A/D conversion results with respect to the same voltage range (Vmin to Vmax) are different in magnitude.